The recently acquired mastery by the author to remove surgically, the still embryonic Thymus for Ambystoma and newts (at stages 28-29 Harrison, at stages 18-19 for Rana) permits the investigation of the role of that gland in the morphogenesis of organs which develop only following the onset of circulation. The preliminary experiments with bilaterally irradiated Thymuses having shown that forelimbs from such embryos do not develop, it is promising to investigate whether this developmental inhibition is attributable to general circulatory failure concomitant with the thymoprivic condition. Whether the loss of limb- forming potential in absence of the Thymus becomes permanent or whether it can be recuperated may be tested either by transplantation of a normal Thymus or by treatment with the several Thymic hormones now available. The recently discovered failure of very young thymectomized Ambystoma larvae to regenerate their limbs is also planned for investigation by similar routine methods of developmental biology. Also proposed is the investigation, by presence/absence methods and by hormonal replacement experiments, whether or not the destructive elimination of appendages (limbs and tails)--the regression effect in denervated, irradiated, hypophysectomized and Cytoxan treated amphibians--can be shown to be, in effect, part and parcel of the working of the immune system of these organisms controlled by the Thymus and the angioblast.